1. Field of the Invention
The present invention relates to an error propagation control method and device for performing error propagation control in multi-level decision feedback equalization (MDFE), and more particularly to an error propagation control method and device suitable for M3DFE.
2. Description of the Related Art
Recently the surface recording density of magnetic disk devices and magnetic tape devices is dramatically increasing. An increase in surface recording density means an increase in transfer speed. If the same signal processing method is used, an increase in transfer speed induces an increase in recording frequency, reaching the limit of recording because of [the limitation of] the write head. Therefore improving signal processing is in progress.
In signal processing, multi-level decision feedback equalization (MDFE) has been proposed. This method prevents a decrease in bit interval and code interference due to a decreased bit interval, therefore the signal-to-noise ratio can be improved.
Also, M3DFE, where a plurality of decision paths of MDFE are set, has been proposed (INTERMAG ""99 Conference, AD-8). With this method, SNR can be improved even more so than MDFE.
FIG. 13 is a functional block diagram of a conventional M3DFE system, and FIG. 14 and FIG. 15 are drawings depicting the path selection sequence thereof.
As FIG. 13 shows, the M3DFE system 90 has a plurality of decision paths 80 and 81 of MDFE. One MDFE (multi-level decision feedback equalizer) 80 is basically comprised of a feedback filter 93, an addition circuit 91, a detector 92 and a buffer memory 94.
The other MDFE (multi-level decision feedback equalizer) 81 has the same structure, comprising a feedback filter 97, an addition circuit 95, a detector 96 and a buffer memory 98. The feedback filters 93 and 97 feedback the output of a decision by the detectors 92 and 96 to the addition circuit 91 and 95. The output of the feedback filters 93 and 97 is changed so as to be the opposite polarity of the input pulse polarity. In other words, the feedback filters 93 and 97 assume that the polarity of the input reproducing signal pulse alternates.
The output polarity of the feedback filter is therefore normally in the opposite direction from the polarity to be input to the forward filter, which is not illustrated, in the previous stage. The addition circuits 91 and 95 determine the difference between the output of the forward filter and the output of the feedback filters 93 and 97. As a result, the output of the addition circuits 91 and 95 become a waveform with the xe2x80x9c0xe2x80x9d level at the center.
Then the detectors 92 and 96 execute binary decision for the output of the addition circuits 91 and 95 by a predetermined threshold value (e.g. xe2x80x9c0xe2x80x9d level). The output of the detectors (decision units) is a binary code series, but is in a four value state because of one bit convolution by the M3DFE circuit.
This M3DFE selects either an MDFE operation for outputting the detection result of one MDFE path, or an M3DFE operation for selecting a detection signal of a path which error signal is smaller according to the detection results of the two MDFE paths when it is judged that error probability is high in the above path. For this a path controller 99 is disposed.
This operation will be described with reference to FIG. 14. operation is different between inside the erasure area and outside the erasure area. At first, in the outside of the erasure area, the detection result of the MDFE path 80 is output via the buffer 94. During this time, the path controller 99 monitors the output Yt (n) of the adder 91, and when the absolute value abs (Yt(n)) becomes smaller than a predetermined level xcex1t, the erasure area starts with this point as the erasure point.
Erasure starts when detecting the input Yt (n) of the decision unit 92 deviates from the ideal sample for a predetermined distance, that is, when detecting the error probability is high. Outside the erasure area, the operation is the same as MDFE, but inside the erasure area, the paths have different decision results from the erasure start point. And error signals for an error path are increased by the feedback filters 93 and 97.
Specifically, a bit flip 82 is set on the other MDFE path 81 so that the decision result of the decision unit 96 on the other MDFE path 81 is inverted by the bit flip 82. Normally an erasure ends when several samples have passed after the erasure point (e.g. length of the buffer 94). This erasure end point is called a xe2x80x9cmergexe2x80x9d.
Therefore different decision results are stored in the buffer memories 94 and 95. The path controller 99 selects a path which has a smaller error signal at the merge point (best path). For example, if the error of the MDFE path 80 is smaller, the detection result of the buffer 94 is output. Whereas if the error of the MDFE path 81 is smaller, the detection result of the buffer 98 is copied to the buffer 94, then the data of the buffer 94 is output.
If a sample which error probability is even higher exists inside the erasure area, as shown in FIG. 15, then the erasure area may be continued after selecting the best path. This continuous point is called a xe2x80x9cresplitxe2x80x9d.
The M3DFE method, which outputs a detection result with less error, has about a 2dB gain compared with the MDFE method.
However, with the conventional M3DFE method, which has no error propagation control function, the error propagates to other data if an error exists, which makes detection impossible. This makes it especially difficult [for M3DFE] to be applied to the magnetic recording field.
An error propagation control method which can be applied to the MDFE method is known, but this cannot be applied to a multi-level decision feedback equalizer having a plurality of paths, even if it can be applied to a single feedback equalizer.
With the foregoing in view, it is an object of the present invention to provide an error propagation control method and device to add the error propagation control function to a multi-level decision feedback equalizer having a plurality of paths.
It is another object of the present invention to provide an error propagation control method and device to implement an error propagation control sequence to apply error propagation control technology, which is applied to a single feedback equalizer, to a multi-level decision feedback equalizer having a plurality of paths.
In order to achieve the above objects, the present invention is an error propagation control method for a multi-level decision feedback equalizer having a plurality of multi-level decision feedback paths comprising: a first error propagation detection step for detecting error propagation of a first multi-level decision feedback path, a second error propagation detection step for detecting error propagation of a second multi-level decision feedback path, a step for performing error propagation control of the first multi-level decision feedback path according to the error propagation detection by the first error propagation detection step for outside the erasure area, and a step for performing error propagation control of a selected path based on the error propagation detection result of the selected path at the erasure end point.
An error propagation control device of the present invention comprises: a first error propagation detector for detecting an error propagation of a first multi-level decision feedback path, a second error propagation detector for detecting an error propagation of a second multi-level decision feedback path, and a control section for performing error propagation control of the first multi-level decision feedback path according to error propagation detection by the first error propagation detector for outside the erasure area and for performing error propagation control of a selected path based on the error propagation detection result of the selected path at the erasure end point.
In an aspect of the present invention, an error propagation detector is set on each path so as to detect error propagation of each path independently, and error propagation of a selected path is controlled at the erasure end point according to the error propagation detection of the selected path so that the error propagation of the selected path can be controlled. Also, error propagation control of a non-selected path having high error probability is inhibited, so unnecessary propagation control can be prevented.
An error propagation control method according to another aspect of the present invention has a step for resetting the above mentioned error propagation detection at the above mentioned erasure start point. The error propagation control device resets the above mentioned error propagation detector at the above mentioned erasure start point.
Since error propagation detection in an unstable state at the erasure start point is reset, error propagation is accurately detected.